Polypropylene homopolymer has many useful applications. However, polypropylene homopolymer alone is often unsuitable for applications that require low melting point and higher flexibility as well as enhanced clarity. Polypropylene random copolymers (RCP) are specially suited for such applications.
Conventional RCPs are typically made through random incorporation of ethylene or other comonomer into polypropylene. The presence of comonomer disrupts polymer stereoregularity and lowers its crystallinity, resulting in lower melting point, lower modulus and higher clarity.
A large number of processes for preparing propylene homo- and copolymers are known in the art. Many different kinds of slurry and gas phase processes can be employed when a supported catalyst is used for polymerization.
One type of propylene polymerization process is a bulk or a slurry process, wherein the reaction takes place in pure monomer or in a reaction medium containing more than 60 weight % of the monomer. The bulk process is carried out in continuously stirred tank reactors (CSTR) or loop reactors. In a loop reactor, the first reaction stage consists of one or two tubular loop reactors where bulk polymerization of homopolymers is carried out in liquid propylene. Prepolymerized catalyst, liquid propylene, hydrogen for controlling molecular weight are continuously fed into the reactor in which polymerization takes place at temperatures of 60-80° C. and pressures of 35-40 bar. The polymer in the liquid propylene inside the loops is continuously discharged to a separation unit, and unreacted propylene is recycled to the reaction medium. Granular product is discharged to a flashing unit for product/monomer separation.
One difficulty associated with slurry processes is granular or fine particle generation. This is especially true for the production of high melt flow rate (MFR) polypropylene.
Random copolymers produced during bulk/slurry polymerizations using hydrocarbon solvents, in particular polymers of high ethylene content and/or low molecular weight, are sticky in the reaction medium. This can cause considerable problems in such bulk/slurry polymerization applications. This problem can be mitigated by operating the polymerization reactor under super critical conditions as disclosed in WO 92/12182, since by nature a super critical fluid has lower solvency to polymer, and nearly unlimited solubility of gaseous components. Simultaneously, the separation of the recycled reaction medium and recovered polymer is simplified under supercritical conditions, because of the energy available in the polymerization product. However, supercritical operation requires handling of high-pressure equipment and is energy intensive and expensive.
Production of high ethylene content and/or low molecular weight polymers also causes difficulty in the operation of conventional flash systems. Such flash systems are highly sensitive to highly soluble polymer fractions. Any non-evaporated liquid in the separation tank risks blocking the device. This is particularly true for cyclone type of devices operated at high pressures.
Processes originally used in the manufacture of polypropylene were based on the use of a hydrocarbon diluent to suspend crystalline polymer particles formed in the process and dissolve the amorphous polymer fraction. Residual catalyst components were deactivated and solubilized by treatment with alcohol, and the deactivated catalyst separated from the diluent by treatment with water. The crystalline polymer product was separated from the diluent by filtration or centrifugation and then dried. The amorphous polymer, which was soluble in the diluent, was separated by evaporation.
What has been referred to as fourth generation polymerization catalysts in Polypropylene Handbook, Edward P. Moore, Jr., Ed., Hanser Publishers, 1996, have led to processes that do not require the use of diluents in the polymerization process by using either liquid or gaseous monomer as the polymerization medium. The stickiness of polymer can be mitigated through reducing the granule swell and improved particle morphology. An example of a polymerization process that incorporates the use of a diluent is shown in U.S. Pat. No. 3,470,143 (Schrage et al.). Specifically, the Schrage patent discloses the use of a fluorinated organic carbon compound as a diluent in polymerizing at least one ethylenically unsaturated hydrocarbon monomer to form an amorphous elastomer. The product can be dried in the form of small particles.
EP 1 323 746 shows loading of biscyclopentadienyl catalyst onto a silica support in perfluorooctane and thereafter the prepolymerization of ethylene at room temperature.
U.S. Pat. No. 5,624,878 discloses the polymerization using “constrained geometry metal complexes” of titanium and zirconium.
There remains a need to increase polymer product quality and process efficiency, particularly processes that reduce slurry polymerization fouling without suffering any substantial loss in polymerization activity. It is particularly desirable to find polymerization processes that use propylene as at least one monomer feed component, and to produce a polypropylene polymer or copolymer type product that can be recovered in particle form. Such a process would also be desirable in the production of propylene polymers with little to no copolymer or propylene-ethylene type polymers that have any of a wide range of ethylene molecules incorporated into the copolymer. Processes that provide for higher flexibility in types of catalyst that can be used, as well as provide copolymers that are very low in crystallinity are especially preferred.